1. Field of the Invention
The present invention relates to a silver recovery device for recovering silver from photographic rinsing water by ion exchange.
2. Description of the Prior Art
To recover silver from the rinse water of a photographic processor, there exist the following available alternatives: electrolytic recovery, metallic replacement, precipitation and ion exchange. It has been established that ion exchange is the least expensive and also the most effective technique for reducing the silver content to a rate that is low enough for effluent water.
Ion exchange can be defined as a reversible exchange of ions between a solid (resin) and a liquid (water containing ionized salts). Silver in the rinsing water is in the ionized form of silver complexes, more particularly silver thiosulfate compounds, e.g. [Ag(S.sub.2 O.sub.3).sub.2 ].sup.-3 and [Ag(S.sub.2 O.sub.3).sub.3 ].sup.-5. Conventionally, to recover the silver, a strong base anion exchange resin is used. In this resin, chloride is the mobile ion and it exchanges with the silver thiosulfate to release a chloride ion while capturing the silver complex, e.g. according to the following scheme: EQU [R].sup.+ Cl+1/3[Ag(S.sub.2 O.sub.3).sub.2 ].sup.-3 .fwdarw.[R].sup.+ [Ag(S.sub.2 O.sub.3).sub.2 ].sup.-3 1/3+Cl.sup.-.
A serious problem encountered with ion exchange of photographic rinsing water is that gelatin clogs the resin and restricts the liquid flow.
It has been proposed to operate with flow and counterflow through the resin. Rinse water flows through resin in a first column to the bottom, the silver becoming captured by the resin and the gelatin becoming physically deposited on the resin. From the bottom of this first column, the water is fed through the bottom of a second column where it vigourously agitates the resin and washes out the entrapped gelatin. After some time the flow through the two columns is reversed so that the second column acts as the silver exchange column while the first column is backwashed. This method is disclosed in SMPTE Journal, Vol. 86, No. 2, February 1977 pages 65 to 68.
A similar method, wherein the rinsing water is used to fluidize a resin bed and wash out the entrapped gelatin is disclosed in the Journal of Imaging Technology, Vol. 10, No. 6, December 1984, pages 244 to 246. Also in this case two resin columns are used in series, the liquid flow being reversed periodically.
The mentioned installations perform well in practice, but are expensive because of the pressurized columns and the valves that are required to reverse the liquid flow. In the case of large scale installations the valves do not raise problems with clogging because they are large and their pneumatic control overcomes occassional blocking.
If this kind of installation is used on a small scale, however, the valves will normally be solenoid-operated and experience shows that a trouble-free operation may raise difficulties in the long run.